In many circuit applications it is necessary to achieve alternating current (AC) coupling of RF signals. This may be necessary to keep the signal centered within the allowable voltage swing range to prevent clipping and/or to eliminate any unwanted direct current (DC) that can cause distortion. This is particularly true in zero intermediate frequency (ZIF) receiver schemes where unwanted DC signal components can cause false carrier signals.
Typically, elimination of the DC components is done by using a coupling capacitor that feeds into a resistive load. However, this technique does not achieve centering of the signal swing ahead of the capacitor. Moreover, for very low frequency coupling, the resistor and/or capacitor values must be large and not practical for IC implementation. In ZIF receiver technology that is presently used, these problems are solved by combining the use of coupling capacitors to eliminate the DC components at the baseband filter output, plus an offset correction circuit ahead of the coupling capacitors to maintain signal swing centering. Two long time constants (.tau.) are generated with this approach namely the coupling capacitor time constant and the offset correction time constants. Each of these have a tendency to create unwanted transient responses when the baseband DC level changes due to varying signal level.
Prior art FIG. 1 shows a block diagram of a typical DC offset correction loop 100. In the implementation shown, the input 101 is a signal current while the output 109 is a signal voltage. The input signal 101 is passed through a baseband filter 103, voltage gain stage 105 and baseband filter 107. A servo loop is created by the addition of the operational transconductance amplifiers (OTA) 111 and 113 and the integrator capacitor 115.
In operation, the output voltage 109 is compared with a fixed voltage reference such as analog ground 117 in the operational transconductance amplifier 111. The difference of the output signal 109 from a reference voltage Vag is amplified into a current signal that is then integrated by the capacitor 115. The resulting voltage from the integration is applied to a second amplifier 113 to amplify the appropriate correction current Icor. Except for any input offset voltage provided by amplifier 111, the capacitor 115 provides virtually infinite gain at DC. This will remove any DC offset relative to analog ground Vag at the output 109.
The offset correction loop generally maintains the output voltage close to the analog ground reference, but as indicated above, only to within the input offset voltage of the error amplifier 111. Hence, without other techniques, it is not practical to obtain an offset voltage below the range of approximately 1 to 5 millivolts (mV) in an integrated circuit (IC) implementation.
In a ZIF IC, it has been necessary to add coupling capacitors between the output voltage 109 and the next stage in order to eliminate the offset of the error amplifier 111 to insure the DC offset is below about 0.1 mV. Additionally, other problems with this technique include the requirement that the operational transconductance amplifier 111 have a very low transconductance on the order of hundreds of pico-Siemens (pS) to tens of nano-Siemens (nS) to realize the required high pass corner of 1 Hz over the range of forward signal path gain. This realization requires relatively large silicon area and its robustness in generating the required output current is questionable.
FIG. 2 shows a graphical representation of a frequency modulated (FM) signal with a single tone that has been frequency translated to a center frequency close to 0 Hz (complex In-Phase and Quadrature-Phase signals), low pass filtered, and corrupted with a DC component at 0 Hz. The spectral components other than the one at 0 Hz represent the desirable Bessel components of the FM signal. The presence of this large DC component in the spectrum can potentially cause self-quieting of the receiver and thus requires nullification. Thus, the need exists to provide an apparatus and method for achieving AC coupling of very low signal frequencies in a relatively small IC die area without coupling capacitors. Also, the need exists to achieve centering of the signal to allow for maximum signal swing.